Interferometers use interfering beams to perform measurements of objects. The interferometers may be broadly classified as equal-path, in which the interfering beams have traversed nearly equal optical distances (e.g., equal to within a few tens of microns), and unequal-path, for which the optical path difference is large compared to the coherence length of visible white light (e.g., greater than 0.05 mm and potentially up to several kilometers). Equal-path systems can be configured to operate with low-coherence (spectrally-broadband and/or spatially extended) light sources. Unequal-path interferometers include, e.g., laser Fizeau, which can be used for testing optical components.
Equal path interferometers are of interest in optical testing, e.g., for separately measuring the front- and back-surfaces of a semi-transparent object. Equal path interferometers can also be used in interference microscopy, where low-coherence halogen lamps and white-light LED's can be used as light sources. For example, interference microscopy designs can be based on path-balanced and dispersion-compensated Mirau, Michelson or Linnik interferometers.